1. Source two USB plugs and a socket, and if required, a nice little case; I butchered one of these because, as a hub, it was crap
(Unless you want to try and cram the cables leading from both plugs into the back of one socket, or to modify a micro USB lead to have two regular USB A plugs on one end. Made this way it can also be used with other devices and a USB charging lead, PSPs and DS-es for example).
2. Solder the green and white wires leading to the socket together, this signals to the phone to draw 1A (it does for a Desire, anyway). Trim the green and white wires leading to the plugs down if you like, they aren't required.
3. Solder the three red wires together, and the three black wires together, into two separate bundles, and insulate them from each other.
4. Close it all back up.
5. Plug both sockets into a computer, a standard micro USB lead into this, THEN the micro USB plug into your phone (after connecting both the other USB plugs to the computer - otherwise the phone will try to draw 1A once you've just got one plug in). All apps -> Settings -> About phone -> Battery should report "Charging (AC)" at the top of the screen. Charges up FAST for me, much quicker than just using a single USB port (500mA). I haven't timed it but it's probably the same as using the wall charger.
Why not just get another wall charger? Some places don't like you plugging your own stuff in, in case it upsets their buildings insurer or it trips the breaker. In this case you are using the existing power supply inside the computer, which is probably tested for electrical safety every twelve months, and would likely be safer for your phone than some cheap POS wall plug from eBay.
Not sure if there is some product like this out there already, but I had the bits laying around so I nicked the idea from the portable hard disks with two USB plugs for extra power.
nb, may not work as well with laptops.
Tested on two computers but still, YMMV.
Why in the Desire section? It's the phone I have, and have tested this on. Mods, please move this if it works for other phones and people would find it useful.